Electromagnetic shielding composite and method for making the same

ABSTRACT

An electromagnetic shielding composite includes a polymer and a plurality of carbon nanotubes disposed in the polymer in a form of carbon nanotube film structure. A method for making an electromagnetic shielding composite includes the steps of: (a) providing an array of carbon nanotubes; (b) drawing a carbon nanotube film from the array of carbon nanotubes; (c) providing a substrate, covering at least one carbon nanotube film on the substrate to form a carbon nanotube film structure; and (d) providing a polymer and combining the carbon nanotube film structure with the polymer to form an electromagnetic shielding composite.

BACKGROUND

1. Field of the Invention

The present invention relates to an electromagnetic shielding compositeand method for making the same.

2. Discussion of Related Art

With the development of information technology, the harmful effects ofelectromagnetic interference, electromagnetic leakage, andelectromagnetic radiation has attracted a lot of attention. And now,metal materials with good conductivity, such as copper and silver, arewidely used as electromagnetic shielding materials. However, the metalmaterials have some disadvantages, such as high cost, high density, easyto oxidize and erode.

Carbon nanotubes (CNTs) are a novel carbonaceous material and havereceived a great deal of interest since the early 1990s. CNTs have alarge ratio of length/diameter (being always more than 1000). Thisspecial structure ensures they have excellent electrical and mechanicalproperties, such as better conductivity than copper among other things.Due to these properties, CNTs have become an important new material foruse in electromagnetic shielding.

Nowadays, the method for using CNTs in electromagnetic shielding is bydispersing the CNTs in polymer to form a composite. But due to theirsmall diameter and large surface energy, the CNTs are easy toagglomerate. A process of surface modification is used to avoidagglomeration. However, this process of surface modification has thefollowing disadvantages among others: firstly, the process results indefects of the walls of the CNTs, such as decreasing the length/diameterratio, which negatively impacts conductivity and other propertiesthereof, and thereby affects the property of electromagnetic shielding;secondly, this method is only suitable for combining CNTs with just afew types of polymers, which restricts the scope of applications; andthirdly, the method of surface modification is expensive.

What is needed, therefore, is an electromagnetic shielding composite andmethod for making the same, with good electromagnetic shieldingproperties, and the method for making the electromagnetic shieldingcomposite is simple and without need of a surface pre-treatment for theCNTs.

SUMMARY

In one embodiment, a method for making an electromagnetic shieldingcomposite includes the steps of: (a) providing an array of carbonnanotubes; (b) drawing a carbon nanotube film from the array of carbonnanotubes; (c) providing a substrate, covering at least one carbonnanotube film on the substrate to form a carbon nanotube film structure;and (d) providing a polymer and combining the carbon nanotube filmstructure with the polymer to form an electromagnetic shieldingcomposite.

Other advantages and novel features of the present electromagneticshielding composite and method for making the same will become moreapparent from the following detailed description of preferredembodiments when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present electromagnetic shielding composite andmethod for making the same can be better understood with reference tothe following drawings. The components in the drawings are notnecessarily to scale, the emphasis instead being placed upon clearlyillustrating the principles of the present electromagnetic shieldingcomposite and method for making the same.

FIG. 1 is a flow chart of a method for making an electromagneticshielding composite, in accordance with a present embodiment.

FIG. 2 shows a Scanning Electron Microscope (SEM) image of a carbonnanotube film structure for the electromagnetic shielding composite, inaccordance with another present embodiment.

FIG. 3 is a structural schematic of an electromagnetic shieldingcomposite using the carbon nanotube film structure of FIG. 2 and madeaccording to the method of FIG. 1.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one preferred embodiment of the presentelectromagnetic shielding composite and method for making the same, inat least one form, and such exemplifications are not to be construed aslimiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe, in detail,embodiments of the present electromagnetic shielding composite andmethod for making the same.

Referring to FIG. 1, a method for making an electromagnetic shieldingcomposite includes the steps of: (a) providing an array of carbonnanotubes, quite suitably, providing a super-aligned array of carbonnanotubes; (b) drawing a carbon nanotube film from the array of carbonnanotubes; (c) providing a substrate, covering at least one of thecarbon nanotube film on the substrate to form a carbon nanotube filmstructure; and (d) providing a polymer and combining the above-describecarbon nanotube film structure with the polymer to form anelectromagnetic shielding composite.

In step (a), a given super-aligned array of carbon nanotubes can beformed by the steps of: (a1) providing a substantially flat and smoothsubstrate; (a2) forming a catalyst layer on the substrate; (a3)annealing the substrate with the catalyst layer thereon in air at atemperature in an approximate range from 700° C. to 900° C. for about 30to 90 minutes; (a4) heating the substrate with the catalyst layerthereon at a temperature in an approximate range from 500° C. to 740° C.in a furnace with a protective gas therein; and (a5) supplying a carbonsource gas to the furnace for about 5 to 30 minutes and growing asuper-aligned array of carbon nanotubes on the substrate.

In step (a1), the substrate can be a P-type silicon wafer, an N-typesilicon wafer, or a silicon wafer with a film of silicon dioxidethereon. Preferably, a 4 inch P-type silicon wafer is used as thesubstrate. In step (a2), the catalyst can, advantageously, be made ofiron (Fe), cobalt (Co), nickel (Ni), or any alloy thereof.

In step (a4), the protective gas can, beneficially, be made up of atleast one of nitrogen (N₂), ammonia (NH₃), and a noble gas. In step(a5), the carbon source gas can be a hydrocarbon gas, such as ethylene(C₂H₄), methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆), or anycombination thereof.

The super-aligned array of carbon nanotubes can, opportunely, have aheight of about 200 to 400 microns and includes a plurality of carbonnanotubes parallel to each other and approximately perpendicular to thesubstrate. The super-aligned array of carbon nanotubes formed under theabove conditions is essentially free of impurities, such as carbonaceousor residual catalyst particles. The carbon nanotubes in thesuper-aligned array are closely packed together by the van der Waalsattractive force.

Step (b) further includes the substeps of: (b1) selecting a plurality ofcarbon nanotube segments having a predetermined width from the array ofcarbon nanotubes; (b2) pulling the carbon nanotube segments at aneven/uniform speed to form the carbon nanotube film.

In step (b1), quite usefully, the carbon nanotube segments having apredetermined width can be selected by using an adhesive tape as a toolto contact with the super-aligned array. In step (b2), the pullingdirection is, usefully, substantially perpendicular to the growingdirection of the super-aligned array of carbon nanotubes.

More specifically, during the pulling process, as the initial carbonnanotube segments are drawn out, other carbon nanotube segments are alsodrawn out end to end, due to the van der Waals attractive force betweenends of adjacent segments. The carbon nanotube film produced in suchmanner can be selectively formed having a predetermined width. Thecarbon nanotube film includes a plurality of carbon nanotube segments.The carbon nanotubes in the carbon nanotube film are mainly parallel tothe pulling direction of the carbon nanotube film.

A width of the carbon nanotube film depends on a size of the carbonnanotube array. A length of the carbon nanotube film can arbitrarily beset as desired. In one useful embodiment, when the substrate is a 4-inchtype wafer as in the present embodiment, a width of the carbon nanotubefilm is in an approximate range from 1 centimeter to 10 centimeters. Athickness of the carbon nanotube film is in an approximate range from0.01 micrometer to 100 micrometers.

In step (c), the substrate can be any kind of substrate necessary toform an electromagnetic shielding material on a surface thereof. In thepresent embodiment, the substrate is a quadrate plastic substrate. Itcan be understood that the size of the substrate can be arbitrarily setaccording to the actual application. When the width of the substrate islarger than that of the carbon nanotube film, a plurality of carbonnanotube films can be covered on the surface of the substrate side byside.

It is noted that because the carbon nanotubes in the super-alignedcarbon nanotube array have a high purity and a high specific surfacearea, the carbon nanotube film is adhesive. As such, the carbon nanotubefilm can adhere to the surface of the substrate directly. A plurality ofcarbon nanotube films can adhere to each surface one after another toform a multi-layer carbon nanotube film structure. The number of thelayers and the angle between the aligned directions of two adjacentlayers are arbitrary and depends on the actual needs/use. The aligneddirections of the two adjacent layers form an angle α, and the angle αis in an approximate range of 0°≦α≦90°. The adjacent layers of thecarbon nanotube film are combined by van de Waals attractive force toform a stable multi-layer film.

It can be understood that alternatively, the substrate can be a framestructure. The carbon nanotube film can adhered, directly, to thesurface of the frame structure and fixed via the periphery of the carbonnanotube film.

Quite usefully, when the carbon nanotube film structure is formed, by aplurality of carbon nanotube films, the carbon nanotube films cross andoverlap each other. In another step, the carbon nanotube film structureis treated with an organic solvent. The organic solvent is volatilizableand can be selected from the group consisting of ethanol, methanol,acetone, dichloroethane, chloroform, and combinations thereof. In thepresent embodiment, the organic solvent is ethanol in the presentembodiment. The carbon nanotube film structure can be treated by eitherof two methods: dropping the organic solvent from a dropper to soak theentire surface of a side of the carbon nanotube film structure orimmersing a frame with the carbon nanotube film structure thereon in acontainer having an organic solvent therein. After being soaked by theorganic solvent, the carbon nanotube segments in the carbon nanotubefilm can at least partially shrinks into carbon nanotube bundles due tothe surface tension created by the organic solvent. Due to the decreaseof the specific surface area via bundling, the coefficient of frictionof the carbon nanotube film is reduced, but the carbon nanotube filmmaintains high mechanical strength and toughness. Further, due to theshrinking of the carbon nanotube segments into carbon nanotube bundles,the parallel carbon nanotube bundles are, relatively, far apart(especially compared to the initial layout of the carbon nanotubesegments) from each other in one layer and at an angle compared with theparallel carbon nanotube bundles in adjacent layers. As such, a carbonnanotube film having a microporous structure can thus be formed (i.e.,the micropores are defined by the spaces/gaps between adjacent bundles).The resulting spaces/gaps of the microporous structure can,beneficially, be in a range of about 100-500 mesh. It is to beunderstood that the microporous structure is related to the number ofthe layers of the carbon nanotube films in the carbon nanotube filmstructure. The greater the number of layers that are formed in thecarbon nanotube film structure, the greater the number of bundles in thecarbon nanotube film structure there will be. Accordingly, thespacing/gaps between adjacent bundles and the diameter of the microporeswill decrease. Further, a carbon nanotube film structure of arbitrarywidth and length can be formed by layers of carbon nanotube filmspartially crossed and overlapped with each other. The width and lengthof the carbon nanotube film structure are not confined by the width andthe length of the carbon nanotube film pulled from the array of carbonnanotubes.

Referring to FIG. 2, a carbon nanotube film structure is acquiredaccording to the present embodiment. The carbon nanotube film structurehas 24 layers of carbon nanotube films crossed and overlapped with eachother. The carbon nanotube segments in each layer of carbon nanotubefilm are aligned and connected end to end, and the alignment of twoadjacent layers is different by 90°. Furthermore, the carbon nanotubefilm structure is treated with ethanol. After being soaked by ethanol,the carbon nanotube segments in the carbon nanotube film can at leastpartially compact/shrink into carbon nanotube bundles due to the surfacetension created by ethanol. Then the parallel carbon nanotube bundlesare, relatively, far apart (especially compared to the initial layout ofthe carbon nanotube segments) from each other in one layer and at anglecompared with the parallel carbon nanotube bundles in adjacent layers.As such, a carbon nanotube film having a microporous structure can thusbe formed. A diameter of the micropores in the microporous structure isin an approximate range from 1 nanometer to 0.5 micrometer. After step(c), a step of removing extra portions of the carbon nanotube film onedges of the substrate is further provided.

In step (d), the polymer can be a solid polymer or a polymer solutionformed by a polymer dissolved in a volatile organic solvent. The solidpolymer can be rubber or plastic. The polymer solution can be epoxypolymer solution or polypropylene solution.

When the polymer solution is combined with the carbon nanotube filmstructure, step (d) further includes the following steps of: (d1)dipping the carbon nanotube film structure prepared by theaforementioned method, directly, in a container with the polymersolution therein for some time in an approximate range from 1 hour to 12hours to make the polymer combine with the carbon nanotube filmstructure; (d2) taking the carbon nanotube film structure out, anddrying the carbon nanotube film structure at a certain temperature toremove the volatilizable organic solvent to acquire an electromagneticshielding composite. The aforementioned drying temperature is in anapproximate range from 80° C. to 120° C.

When the solid polymer is combined with the carbon nanotube filmstructure, step (d) further includes the following steps of: (d3)covering the solid polymer on the carbon nanotube film structure; and(d4) heating the solid polymer and the carbon nanotube film structure,under a certain pressure, to a certain temperature to make the solidpolymer combined with the carbon nanotube film structure; (d5) coolingthe solid polymer and the carbon nanotube film structure to acquire anelectromagnetic shielding composite. The heating temperature can behigher than a glass transition temperature of the polymer for 20-50° C.and lower than the decomposition temperature of the polymer, and thepolymer is liquid and can flows at that temperature. The aforementionedpressure can be in an approximate range from 3 atmospheres to 10atmospheres.

It can be understood that a substrate necessary to make anelectromagnetic shielding composite is chosen, then a carbon nanotubefilm structure is formed on the substrate, the substrate and the carbonnanotube film structure are dipped in a polymer solution for some timeor a polymer is covered on the carbon nanotube film structure, thepolymer and the carbon nanotube film structure are heated, under acertain pressure, to a certain temperature to combine the polymer withthe carbon nanotube film structure; or the substrate with the CNT filmstructure is removed via dissolving or stripping, then the carbonnanotube film structure is dipped in a polymer solution for some time orthe carbon nanotube structure is disposed between two layers of solidpolymer, and finally, heating the polymer and the carbon nanotube filmstructure, under a certain pressure, to a certain temperature to makethe combination thereof.

In the present embodiment, the carbon nanotube film structure and thesubstrate are stripped after the treatment of the substrate with thecarbon nanotube film structure formed thereon by ethanol. Then thecarbon nanotube film structure is dipped in an ethanol solution withepoxy polymer therein for 5 hours. Finally, the carbon nanotube filmstructure is taken out, and dried in a temperature of 80° C. to acquirean electromagnetic shielding composite.

Referring to FIG. 3, an electromagnetic shielding composite 10 preparedby the aforementioned method is shown. The electromagnetic shieldingcomposite 10 includes a polymer 14 and a plurality of CNTs, and the CNTsare distributed in the polymer 14 in a form of a carbon nanotube filmstructure 12. The carbon nanotube film structure 12 can be a layer ofcarbon nanotube film or multiple layers of carbon nanotube film. Whenthe carbon nanotube film structure 12 is a single layer of carbonnanotube film, the carbon nanotube film structure 12 is a carbonnanotube film with a certain width formed by a plurality of carbonnanotube bundles aligned in a same direction and connected end to end.When the carbon nanotube film structure 12 has multiple layers of carbonnanotube film, the a plurality of carbon nanotube films can beoverlapped with each other and the angle between the aligned directionsof two adjacent layers may be arbitrarily set as desired. The angle αbetween the aligned directions of two adjacent layers is in anapproximate range from 0° to 90°. The carbon nanotube film includes aplurality of carbon nanotube bundles in a preferred orientation. Carbonnanotubes bundles in two adjacent layers are crossed with each other toform a microporous structure. A diameter of the micropores is in anapproximate range from 1 nanometer to 0.5 micrometer. The polymer 14 iscovered on the carbon nanotube film structure 12 or filled into themicropores of the carbon nanotube film structure 12.

Compared to the conventional electromagnetic shielding composite andmethod for making the same, the electromagnetic shielding composite andmethod for making the same according to the present embodiments have thefollowing virtues: firstly, the electromagnetic shielding composite isprepared via the carbon nanotube film structure being combined with thepolymer and thus avoiding the problem of dispersing the carbon nanotubesin the polymer; secondly, the surfaces of the carbon nanotubes in theelectromagnetic shielding composite are not treated with a process ofsurface modification and the method for making the electromagneticshielding composite is low cost, simple, and environmentally safe; andthirdly, the content of CNTs in the electromagnetic shielding compositeis not limited and many different polymers can be chosen to combine withthe carbon nanotube film structure, and finally an electromagneticshielding composite with excellent electromagnetic shielding propertyand other properties can be acquired.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the invention. Variations maybe made to the embodiments without departing from the spirit of theinvention as claimed. The above-described embodiments illustrate thescope of the invention but do not restrict the scope of the invention.

1. A method for making an electromagnetic shielding composite, themethod comprising the steps of: (a) providing an array of carbonnanotubes, a substrate, and a polymer; (b) drawing a carbon nanotubefilm from the array of carbon nanotubes, the carbon nanotube filmcomprising a plurality of carbon nanotubes joined end to end by van derWaals attractive forces; (c) covering at least one carbon nanotube filmon the substrate to form a carbon nanotube film structure, extendingdirections of the carbon nanotubes in the carbon nanotube film structureare parallel with the substrate; and (d) combining the carbon nanotubefilm structure with the polymer to form an electromagnetic shieldingcomposite.
 2. The method as claimed in claim 1, wherein the polymer is asolid polymer, and the step of combining is executed by covering thesolid polymer on the carbon nanotube film structure and heating thesolid polymer and the carbon nanotube film structure under a certainpressure to a certain temperature, to combine the solid polymer with thecarbon nanotube film structure to form the electromagnetic shieldingcomposite, wherein the certain temperature makes the solid polymerchange into a flowing liquid.
 3. The method as claimed in claim 1,wherein the polymer is a polymer solution comprising the polymerdissolved in a volatile organic solvent, and the step of combining isexecuted by dipping the carbon nanotube film structure directly into acontainer with the polymer solution therein to combine the polymer inthe polymer solution with the carbon nanotube film structure, afterthat, taking the carbon nanotube film structure out and drying thecarbon nanotube film structure at a certain temperature to remove thevolatile organic solvent to form the electromagnetic shieldingcomposite.
 4. The method as claimed in claim 1, wherein step (c) furthercomprises a step of separating the substrate from the carbon nanotubefilm structure via dissolving or stripping.
 5. The method as claimed inclaim 1, wherein a plurality of the carbon nanotube films overlap toform the carbon nanotube film structure, and the carbon nanotube filmstructure adheres to the substrate.
 6. The method as claimed in claim 5,wherein after the formation of the carbon nanotube film structure on thesubstrate, a process of treating the carbon nanotube film structure withan organic solvent to form a plurality of micropores is furtherprovided.
 7. The method as claimed in claim 6, wherein the organicsolvent is selected from the group consisting of ethanol, methanol,acetone, dichloroethane, and chloroform.
 8. The method as claimed inclaim 2, wherein the pressure is in a range from about 3 atmospheres toabout 10 atmospheres.
 9. The method as claimed in claim 2, wherein thetemperature is higher than a glass transition temperature of the polymerby 20-50° C. and lower than the decomposition temperature of thepolymer.
 10. The method as claimed in claim 1, wherein the carbonnanotubes in the carbon nanotube film structure are parallel with eachother.
 11. A method for making an electromagnetic shielding composite,the method comprising the steps of: (a) providing an array of carbonnanotubes, a substrate, and a polymer; (b) drawing a carbon nanotubefilm from the array of carbon nanotubes; (c) repeating the step (b) tofabricate a plurality of carbon nanotube films; (d) crossing andoverlapping the plurality of carbon nanotube films with each other toform a carbon nanotube film structure on the substrate; (e) forming aplurality of micropores in the carbon nanotube structure by using anorganic solvent; and (f) combining the carbon nanotube film structurewith the polymer to form an electromagnetic shielding composite.
 12. Themethod as claimed in claim 11, wherein the polymer is a solid polymer,and the step of combining is executed by covering the solid polymer on asurface of the carbon nanotube film structure and heating the solidpolymer and the carbon nanotube film structure under a certain pressureto a certain temperature, to combine the solid polymer with the carbonnanotube film structure and form the electromagnetic shieldingcomposite.
 13. The method as claimed in claim 11, wherein the polymer isa polymer solution comprising the polymer dissolved in a volatileorganic solvent, and the step of combining is executed by dipping thecarbon nanotube film structure directly into a container with thepolymer solution therein to combine the polymer in the polymer solutionwith the carbon nanotube film structure, and then removing the carbonnanotube film structure from the polymer solution and drying the carbonnanotube film structure at a certain temperature to remove the volatileorganic solvent and form the electromagnetic shielding composite. 14.The method as claimed in claim 11, wherein an angle between adjacentcarbon nanotube films in the carbon nanotube film structure is in arange from about 0° to about 90°.